1. Field of the Invention
The present invention relates to temperature compensation in a helix resonator.
2. Description of Related Art
It is known that the inner conductor of a helix resonator is wound into a cylindrical coil, and the outer conductor consists of a conductive surface which covers the cylindrical coil. At the resonant frequency, TEM vibration is formed along the longitudinal axis of the resonator. The signal enters the cylindrical coil at its one end, and the other end may be either open or short-circuited. If the other end is open, the helix resonator is equivalent to a quarter-wave coaxial resonator, and if the other end is short-circuited, the helix resonator is equivalent to a half-wave coaxial resonator. By regulating a suitable tuning screw in the resonator structure, the capacitance between the coil and the shield can be adjusted to form an LC series resonance circuit. Usually a plurality of resonators are coupled together in such a manner that a filter having the desired properties is obtained for use, for example, in a radio receiver. Owing to their relatively small size and tunability, helix resonators are highly usable in duplex filters, especially within a frequency range of 100-1000 MHz. Temperature stability constitutes a basic problem in state-of-the-art helix resonators. The stop-band and pass-band frequencies of a duplex filter must not change, for example under the effect of the temperature. Therefore the helix resonators in a duplex filter should be temperature compensated, i.e. their resonant frequency must not vary as a function of the temperature. In applications in which the variation of ambient temperature is wide, substantial deviations in the average frequency of a helix resonator are to be expected. A typical example of such an application is the duplex filter used in mobile telephones. In the state of the art, frequency deviation caused by a change in the temperature has been compensated in various ways. It is possible to use precision components the properties of which are very little affected by temperature changes. However, the use of such components makes the resonator very expensive. Another method is to make resonators tunable over so wide a range that extensive temperature deviations from the average frequency can be allowed. This method is, however, less desirable, since it is carried out at the expense of selectivity. In certain applications, improvement of temperature sensitivity takes place at the expense of tuning sensitivity.
U.S. Pat. No. 4,205,286 describes a temperature-stabilized helix resonator. In this construction the inner conductor is wound around a two-part frame, in which the parts of the frame are coaxial and successive, and the lower part has a greater diameter than the upper part, and the lower part and the upper art are interconnected by means of a flexible joint which allows the parts to move in relation to each other as the temperature changes. Inside the smaller-diameter upper part there extends an adjusting screw, which serves as a tuning element and is supported on the one hand by a threading in the upper part and on other hand by the cover of the shield, with the help of a locking nut. As the ambient temperature changes, the joint between the upper part and the lower part enables these parts to move in relation to each other, but so that the distance of the tuning screw from the conductor coils in the upper part always remains the same, whereupon the capacitive coupling also remains the same regardless of the ambient temperature. The construction of the temperature-stabilized resonator described in this patent application is quite cumbersome and expensive to manufacture, and is rather large in size and has a rather low Q-value, and thus it is suitable for use at rather low frequencies, approx. 100-200 MHz.
Also known is a temperature compensation method in which plastic bonds are injection-molded to the cover of the helix resonator shield. Such a bond comprises one or more projections oriented towards the resonator axis from the cover of the resonator shield, one end of the projections being, as mentioned above, fixed to the resonator shield and the other end extending in part over the topmost turns of one or more resonators in such a manner that the conductor of the resonator coil is in part or entirely inside these projections. Instead of projections it is possible to use one ring-like cylindrical piece, one end surface of which rests tightly against the cover of the resonator shield, and the topmost turns of the resonator coil are within this cylindrical piece. When the temperature increases, the distance of the open end of the resonator from the shield cover changes, and owing to the thermal expansion the length of the coil and the pitch of the turns change. By selecting a suitable material for the projections, an attempt can be made to compensate for the above-mentioned changes. In practice such temperature compensation is undercompensated in character, and this means that the frequency will change somewhat as a function of the temperature. Temperature compensation can be corrected by shifting the undercompensation in the direction of overcompensation to a suitable extent so that, as the temperature changes, the result will, nevertheless, be precise temperature compensation and the frequency will not change as a function of the temperature. The methods of correction have included bringing the open end of a helix resonator closer to the cover of the upper side, or reducing the pitch of the helix resonator, i.e. the distance between the turns, in the area of the abovementioned bonds, or the temperature coefficient of the plastic can be increased.
Bringing the open end of the helix resonator closer to the cover will be helpful only to a certain limit, i.e. the temperature compensation will no longer change in the overcompensated direction even if the resonator end is brought infinitely close to the cover. Bringing the open end of a helix resonator infinitely close to the cover also involves another disadvantage, i.e. the risk of electric breakdown, and such breakdown is possible especially at high voltage levels. It should also be noted that, after a certain optimum distance, the Q-value of the resonance circuit will drop the more the closer to the resonator shield cover the open end of the helix resonator is brought.
As mentioned above, a resonator undercompensated with respect to the temperature can be shifted in the overcompensated direction by reducing within the bound part the pitch of the helix resonator, i.e. the distance between the turns. A practical limit to this method is set by the fact that the turns must not touch each other, and since the turns are in practice already very close to each other the leeway for reducing the distance is very slight. A third possibility in shifting in the overcompensated direction is to increase the temperature coefficient of the plastic, but this is limited by the fact that the number of plastics which can be used is small, since the plastic is required to have also properties other than good temperature properties, and therefore the number of temperature coefficients usable is limited.